It is useful in many communication systems, for example paging systems, to know the location of the portable units or pagers which are operating within the system. The location or position information of the remote units for example can be used for registration and directed delivery of messages to the units or to locate a user in case of an emergency situation.
Prior schemes for locating portable communication devices have relied on one of several possible technologies, all having disadvantages for portable communication unit applications such as paging systems.
In one prior art system, the system computes arrival times of portable-to-base transmissions using direct frequency spread spectrum techniques. The position of the portable unit can be computed when arrival times at three or more receiver sites are known. The main disadvantage of this system is that it requires the use of complex spread spectrum transmitters at the portable device, and must operate in the Industrial, Scientific, and Medical (ISM) bands and accept any interference present in these frequency bands which are unlicensed.
In a second prior art system, the portable units incorporate a LORAN or GPS (global positioning system) receiver to compute its position. The portable units then report their position information to the system. The main disadvantage with this system is that it requires the added cost of integrating a LORAN or GPS receiver in the portable units, which adds further cost and complexity to the portable units. Not to mention that it also forces the portable units to be larger in size in order to accommodate the additional LORAN or GPS circuitry.
A third prior art communication system uses direction finding equipment at each base site in order to fix the location of the portable unit. The position of the portable units are computed by triangulation when two or more base stations receive the same signal. The disadvantage to this system is that the portable units have to transmit for long time periods at high power levels in order to enable accurate measurements of angles by the base stations. Such high power and long duty cycle transmissions would not be effective for small portable communication device applications (e.g., two way paging, etc.) since battery life in these applications is very important.
A fourth prior art communication system known as very high-frequency omnidirectional range (VOR) which is used primarily as a navigation aid for aircraft compares the phase of a fixed and rotating signal to compute its angle with respect to a transmitter station. In a VOR system, a transmitter emits a (variable) modulation whose phase relative to a reference modulation is different for each bearing of the receiving point from the station. A main drawback to a VOR system for use in portable communication applications is that the VOR transmitter(s) have to provide both a reference modulation signal and a variable modulation signal. The typical radio frequency (RF) bandwidth required for a VOR system is around 25 kilohertz (Khz). Such a large bandwidth requirement would be unacceptable for use in a two-way paging or other similar type of portable subscriber application. A need thus exists for a communication system and device which can provide for low cost location of communication devices such as pagers, radiotelephones, etc.